1. Field of the Invention
The present invention relates to a luminescent device using a luminescent element having a film comprising an organic compound which can give fluorescence or luminescence by the application of an electric field (hereinafter referred to as an “organic compound layer”), and a process of manufacturing the same.
In the present invention, the luminescent element means an element wherein an organic compound layer is arranged between a pair of electrodes, and the luminescent device means an image display device or a light-emitting device using the luminescent element. The category of the luminescent device includes all of modules wherein a connector such as a flexible printed circuit (FPC), a tape automated bonding (TAB), or a tape carrier package (TCP) is attached to the luminescent element, modules wherein a printed circuit is attached to the tip of the TAB tape or the TCP, and modules wherein an IC (integrated circuit) is mounted directly to the luminescent element in a chip on glass (COG) manner.
2. Description of Related Art
Luminescent elements using an organic compound as a luminescent body and having characteristics such as thin and light bodies, high-speed responsibility and the capability of being driven by low DC voltage are expected to be applied to flat panel displays in the next generation. Particularly a display device wherein luminescent elements are arranged in a matrix form is superior to conventional liquid crystal display devices in wide field angle and the easiness of being watched.
The luminescent mechanism of a luminescent element is said to be based on the following: by applying voltage to its electrode-pair through its organic compound layer, electron injected from the cathode and holes injected from the anode are recombined in the luminescent center of the organic compound layer to produce molecular excitons, and energy is emitted when the molecular excitons return to their ground state. As excitation state, singlet excitation and triplet excitation are known. It is considered that luminescence can be caused whether the molecular excitons undergo the singlet excitation or the triplet excitation.
In a luminescent device formed by arranging such luminescent elements in a matrix form, a driving method called passive matrix driving (simple matrix type) or active matrix driving (active matrix type) can be used. However, when pixel density increases, it is considered that the active matrix type, wherein a switch is fitted to each of pixels (or dots), is more profitable since this type device can be driven at a lower voltage.
In the manufacture of a luminescent device of the active matrix type, a thin film transistor (hereinafter referred to as a TFT) is formed on an insulating surface, an interlayer dielectric is formed on the TFT, and then an anode of a luminescent element electrically connected through the interlayer dielectric to the TFT is formed. For the material which makes the anode, a transparent conductive material having a large work function is suitable. A typical example thereof is indium tin oxide (ITO).
Furthermore, an organic compound layer is formed on the anode. The organic compound layer includes a hole injection layer, a hole transport layer, a luminescent layer, a blocking layer, an electron transport layer, an electron injection layer and so on. The organic compound layer may have a monolayer structure or a multilayer structure wherein the above-mentioned layers are combined.
After the formation of the organic compound layer, a luminescent element is formed by forming a cathode. In many cases, the material of the cathode is any one of metals having a small work function (typically, metals belonging to the I or II group in the periodic table) or an alloy containing these metals.
An insulating layer made of an organic resin material is formed to cover the edge of the anode and prevent a short circuit between the anode and the subsequently-formed cathode. A luminescent element as described above is formed in each pixel. These luminescent elements are formed in a pixel section, whereby an active matrix type luminescent device is manufactured.
Since the transparent conductive film which has been used so far as the anode can transmit visible rays, this film can transmit light generated in the luminescent layer. However, this film has a drawback that the resistivity thereof is larger than that of metals. As the resistance of the film becomes larger, the injection of carriers becomes more difficult. Therefore, a problem that the carrier-recombining ability of the luminescent element deteriorates occurs.